Plastic coils are well known through their several uses, for example to hingedly bind the pages of a book or booklet. The coils used this way are semi-rigid plastic wires forming a cylindroid helix of constant diameter and pitch. A particular advantage of this form of plastic coil binding element is that it is not expensive to produce, while being sturdy, offering a good aesthetic appearance and allowing an efficient and easy-to-use binding element for the book pages bound in this way.
A known method of producing plastic coils comprises using a conveyor line for a thermoplastic wire in which the wire is heated so as to achieve a semi-viscous pliable state, and then fed into a helical channel in which its selected helical form is achieved by cooling the channel so that the thermoplastic wire will solidify and recover its semi-rigidity in the form of a coil shaped according to the helical channel of constant selected pitch and diameter. The cooling is achieved by any suitable means of cooling the thermally conducting channel, whereby the plastic wire will be cooled by heat transfer therethrough.
The main problem associated with this conventional method for forming plastic coils is that the coil discharged by the helical channel is imparted with a rotative movement about its longitudinal axis, in addition to the desired output discharge translational movement along this same longitudinal axis. Thus, the plastic coil wire cannot be wound around a spool for storing and carrying purposes, because of the coiled wire rotating on itself which would result in an undesirable twisting of the plastic coil on the spool. The usual way to obviate this problem is to cut the coiled wire at regular--and relatively short--lengths, for example at every three feet. This effectively prevents the coils from twisting on themselves, but prevents the coils from being stored, carried and sold as a long, unitary coil. Not only is it less interesting to sell the coils in short lengths, but also to cut the plastic coil is likely to result in an important waste of plastic coil material. Indeed, if the length at which the coil is cut (e.g. three feet) is not a multiple of the effective used lengths of the coils (e.g. to bind books having a length of two feet), then the extra coil material (e.g. one foot long for each three feet coil) must be discarded and is wasted.